Venus will transit the face of the Sun on June 5, forming a small black silhouette against the brilliant Sun. For most of the United States, the transit will be in progress as the Sun sets. Only Alaska and Hawaii will see the entire event. This is the last Venus transit until 2117. Although it is visible to the unaided eye, you need eye protection to view it safely, such as dark welder's glass. [Sun image: NSO]

Today, we know the distances to the Sun and planets to within a few miles — a level of precision that lets us send probes to any target in the solar system. In the not-too-distant past, though, distance estimates varied by millions of miles. It took a precise ruler to turn the estimates into exact numbers.

One way in which astronomers developed such a ruler was by watching transits of Venus — passages of the planet across the face of the Sun.

They journeyed around the world to see the transits from different angles. They measured the precise timing of a transit, and tracked Venus’s path across the Sun. Comparing the measurements from different locations allowed them to triangulate Venus’s position. That revealed the distance to Venus and, more importantly, to the Sun — the basic ruler for measuring all solar-system distances.

Transits are rare, though. There were only two in each of the 18th and 19th centuries, and none in the 20th. There are two in this century — one was eight years ago, and another is coming up tomorrow; more about that on our next program.

And traveling to remote destinations to view the transits was dangerous business. One astronomer, for example, was accused of witchcraft and narrowly escaped execution.

Yet the astronomers persevered, and their results paid off. They developed the basic ruler for measuring astronomical distances — making it possible to journey to the worlds of our solar system.